1. Field of the Invention
The invention relates to a circuit configuration for color decoding and decimation for a video signal. Such a circuit configuration is particularly suitable for inserting a reduced picture into a main picture or parent picture, i.e. so-called picture-in-picture insertion.
Picture-in-picture insertion requires that the picture to be inserted is reduced from its original, input size to the picture size to be inserted. In other words, the picture signal is to be decimated. Moreover, in the course of the picture processing, it is expedient for the brightness and color signal components to be processed separately. These signal components are combined in the video signal at the receiving end. The luminance and two chrominance signal components are obtained from the video signal, i.e. the color decoding, is carried out prior to the decimation.
A prior art circuit configuration for picture-in-picture insertion is described in the SIEMENS product description "ICs for Entertainment Electronics; Picture-in-Picture System," Issue May 1991, in particular page 8 and pages 42-43. A digitally operating color decoder generates a digital luminance signal Y and two digital chrominance signals U, V which are fed to a picture-in-picture processor. The latter contains, on the input side, a decimator which decimates the picture to be inserted with regard to its size. The decimated picture is then buffer-stored in a frame memory. A commercially available module which is not necessarily adapted to the specific application of picture-in-picture insertion is proposed as color decoder. Therefore, the signal components Y, U, V with the largest possible signal bandwidth in each case are fed to the input side of the decimation device. For the purpose of decimation, horizontal and vertical averaging of pixels is carried out for each signal component. For picture-size reduction by the factor 1/9, 9 pixels are averaged to form a single pixel for the picture to be inserted.
As a result of the decimation by means of averaging, the sampling rate of the picture to be inserted is reduced. In order to satisfy the sampling/Nyquist theorem after decimation as well, thereby preventing interfering effects in the decimated picture, corresponding band limiting of the signal components to be decimated is necessary. For this purpose, the averaging operation does actually include low-pass filtering, but the passband thereof is indeterminately delimited such that the decimated picture nevertheless contains interfering effects.
An example of a conventional digital color decoder is described in Fernseh- und Kino-Technik, Ton Nillesen: "Digitaler TV-Farbdecoder mit Zeilenfrequenzverkopplung" [Digital TV Color Decoder With Line Frequency Locking], No. 4/1986, pages 141 to 146. In order to keep the bandwidth of the color-decoded output signals Y, U, V as large as possible, the color decoder has high-quality filters on the output side, namely a respective low-pass filter in the path of the chrominance signals, which filter out the respective mixed products at multiples of the carrier frequency, and a notch filter in the path of the luminance signal component, which filters out the color subcarrier. The entire picture-in-picture system thus has a relatively high outlay on circuitry. A fully integrated implementation of a color decoder and picture-in-picture processor on a single integrated circuit chip would be correspondingly complex.